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Astrophotography

A Solar Observing Refresher Course

Solar filters are typically made with a specially coated Mylar or glass substrate mounted in a cell that fits snugly over the front of the telescope. Such filters offer safe white-light views of the Sun, revealing sunspots, faculae, limb darkening, and a host of lesser features.

Sky & Telescope photo by Craig Michael Utter.

Viewing the Sun provides an enjoyable way to supplement the usual nighttime observing activities, but you should be aware of the potential for serious injury and take precautions to ensure your safety and the safety of others. (See "Solar Filter Safety.") Viewing the Sun also demands extra vigilance when it comes to equipment. Never leave a telescope or binoculars unattended, especially when children are about. It takes only a moment of inattentiveness to create a dangerous situation.

Minimalist Sun-Gazing

Welder's glass of shades 12 through 14 are popular and safe solar filters, easily obtained at welding-supply outlets. Most observers prefer shades 13 or 14; the solar image through a number-12 filter is uncomfortably bright.

Sky & Telescope photo by Chuck Baker.

The Sun is one of the few objects that display a rewarding amount of detail without a telescope or even binoculars. The only equipment you need is an appropriate filter — a piece of No. 14 arcwelder's glass is the traditional choice. This safe filter material is available at any welding-supply store in convenient 4-inch-wide pieces that allow viewing the Sun with both eyes. Although welder's glass imparts a green hue to the Sun, one of these economical filters might be all you ever need for casual observing. But while welder's glass provides satisfactory naked-eye views of the Sun, its poor optical quality makes it unsuitable for use with binoculars and telescopes.

The part of the Sun that we actually see is a layer called the photosphere. It is the nearest thing the Sun has to a "surface." The first features you are likely to notice on the photosphere are large sunspots.

The solar surface, seen in white light, includes sunspots, faculae, prominences, and other features associated with the magnetic fields that result from the motion of ionized matter in the Sun's interior.

Courtesy Don Davis.

Spots of this size are fairly common when the Sun is active, and occasionally several groups are visible at the same time. Tracking the visibility of large spots provides an interesting project for naked-eye observers. In addition to sunspots, look for decreasing brightness toward the edge of the Sun's disk. This limb darkening is the result of looking through a progressively thicker cross section of the darker, cooler upper photosphere near the Sun's limb.

Sun and Telescope

Closeup of a sunspot. This image, taken by the Swedish 1-meter Solar Telescope on La Palma in the Canary Islands, resolves the filaments making up the spot's penumbra with a resolution of 0.1 arcsecond, courtesy of an advanced adaptive-optics system. The black-and-white images have been colorized here to emphasize contrasts. The smallest resolved structures are about 90 kilometers across.

Courtesy Royal Swedish Academy of Sciences.

When it comes to outfitting optical instruments for solar viewing, a number of excellent options are available. However, there is one type of filter that is very dangerous: the eyepiece Sun filter. These were once commonly supplied with imported telescopes and consist of a piece of dark glass mounted in a cell that screws into the bottom of an eyepiece. The heat from the Sun concentrated by a telescope can shatter these filters without warning. Today's advice is to destroy these filters to ensure they can cause no harm.

For safe viewing, most observers choose either a glass or Mylar solar filter mounted in a cell that fits securely over the front aperture of a telescope. Such filters are made with light-rejection coatings that allow only a fraction of a percent of the Sun's light to pass. This style of filter protects not only your eyes but your equipment too, since the potentially harmful heat of the Sun never enters the telescope.

Glass solar filters generally produce a yellow or orange Sun, while Mylar filters usually yield a blue image. Aesthetics aside, there are other differences to consider. Mylar filters tend to offer better contrast between the solar disk and bright faculae surrounding active regions. However, Mylar's blue-tinted image also suffers more from scattered light and atmospheric dispersion than the orange image produced by a glass filter. Somewhat better sunspot detail is seen with a glass filter, but faculae are usually rendered all but invisible. Although these filters are more alike than different, it is probably best to give some thought to what you most want to see before purchasing a filter for your telescope.

Various types of solar filters are helping increase the popularity of solar observing. Here, Sky & Telescope staff members demonstrate several different ways of safely studying the Sun.

Sky & Telescope: Craig Michael Utter.

Telescopic solar observing is pretty straightforward since vendors make filters sized to fit most popular instruments. Simply attach your filter to the front of the tube so that it cannot fall off, and you're in business. Don't forget to make sure that your telescope's finderscope is capped at the objective end or, better yet, removed completely. Aiming the telescope without a finder might seem problematic but it is quite simple. Just move the telescope around until its shadow is minimized, at which point the Sun should be within the field of a low-power eyepiece.

For most visual astronomy, bigger is better — the larger the scope, the more light collected and the greater the theoretical resolution. However, when it comes to solar observing, the playing field is tipped in favor of smaller scopes. Light-gathering is not an issue since we are trying to dim the Sun's intense glare, but what about resolution? Here again, the advantages of a large instrument are essentially neutralized by atmospheric turbulence. Daytime seeing is rarely steady enough to permit the maximum resolution of even a 4-inch telescope.

Glass and Mylar filters can also be used with binoculars. In addition to making it possible to view small sunspots, binoculars will show the limb darkening with greater ease than the naked eye alone. You can purchase filters for many binocular sizes, and you can even make your own from Mylar solar-filter material available from several vendors. Make sure filters are firmly affixed so that they will not fall off or blow away in a gust of wind.

Solar Projection

Small telescopes are especially suited to the solar-projection method. Seen here is a simple projection system made from a cardboard box with a piece of white paper as a projection surface. A surprising amount of detail can be seen with this setup.

Sky & Telescope photo by Craig Michael Utter.

One method of solar observing that dispenses with filtration altogether is solar projection. (See "Observing The Sun By Projection.") An eyepiece is placed in the telescope's focuser and used to project an image of the Sun onto a convenient flat surface. Telescopes with folded light paths, such as Newtonians or Schmidt-Cassegrains, are not recommended since the converging beam of light can produce enough heat to damage internal components.

When it comes to eyepieces for projecting the Sun's image, the much-maligned Huygenian design is a good choice because it does not contain cemented elements that can be damaged by the Sun's intense heat. Most solar projection is done onto white paper or card stock. But no matter how white the screen, it must be adequately shaded from direct sunlight and other extraneous light in order for the viewer to see the finest details in the solar image. This powerful technique enables a 4-inch telescope to produce a usable image of the Sun 30 inches across. The size and brightness of the Sun's image depend mainly on the distance between the eyepiece and the viewing surface — the farther away it is, the larger and dimmer the image.

The Spotted Sun

Sunspots are cooler regions of the solar surface caused by intense localized magnetic fields that bring the upward convection of internal material to a virtual standstill. Although they appear almost black, this is merely a contrast effect. If it were possible to place a modest-size sunspot into the night sky, it would shine 10 times brighter than the full Moon!

Even the casual observer will soon learn that sunspots come in a wide variety of shapes and sizes. While the simplest sunspots are isolated dark areas, larger spots are quite dramatic. Complex spots feature a dark central region called the umbra surrounded by a gray penumbra. The penumbra normally appears as a smooth fringe, but under steady seeing conditions it may exhibit radial patterns or knots of light and dark. During those fleeting moments of good seeing you may also see tiny circular sunspots 2 arcseconds in diameter or smaller. These are called pores. Sometimes they erupt into full-fledged spots but usually they simply disappear — sometimes after a lifetime of only a few minutes.

The Wilson effect, which is greatly exaggerated in this diagram, shows how the umbra and penumbra of a typical sunspot become more and more symmetric as they near the center of the Sun's disk.

Sky & Telescope illustration.

Most sunspots are associated with groups that can change dramatically in a matter of hours. These groups usually consist of a large "flagship" spot, surrounded by several smaller ones. Others can have a pair of large spots accompanied by a retinue of smaller spots and pores arranged in an arc or a line. Normally the large pair will have opposite magnetic polarity — one positive and the other negative.

Sketching sunspots with a pencil and paper can be a rewarding way to follow their evolution. In the same way that drawing the planets sharpens your observing skills, so will regularly recording the Sun's appearance. You can follow the complex ways sunspot groups change with time, and you might even come to regard some active regions as old friends as you watch them disappear beyond the Sun's western limb and reappear on the eastern limb two weeks later. Spots near the Sun's limb sometimes appear like shallow depressions on the solar surface. This is the so-called Wilson effect, named for the 18th-century Scottish astronomer Alexander Wilson, who first called attention to the phenomenon.

More Solar Sights

This sketch by Jeffery Sandel shows a complex sunspot group. Unlike the traditional method wherein the sunspots' positions are plotted and their outlines traced directly on the projected solar image, he draws them freehand on a blank 8-inch-diameter circle. This results in exaggerated sunspot sizes.

Courtesy Jeffery Sandel.

While sunspots are visually the most obvious features on the Sun's disk, careful inspection will reveal that most of them are surrounded by brighter regions called faculae. Limb darkening really helps faculae stand out against the photosphere. Another delicate sight is solar granulation, which imparts a sandy texture to the photosphere. Granulation is caused by the convection of material from the Sun's interior to the surface. Individual granules are only a few arcseconds across and therefore require good seeing to be visible. Under poor seeing conditions, granulation may appear as a much coarser mottling across large areas of the photosphere. It takes only minutes for individual granules to appear, dissipate, and be replaced by new ones.

The solar viewing I've described above is known as white-light observing. If you find Sun-gazing to your liking, you may choose to investigate more advanced forms of observation that use special filters to isolate portions of the spectrum for spectacular views of a wide range of phenomena. Coronagraphs, hydrogen-alpha filters, and other observing gear are available — but at a significantly greater cost than the simple filters needed for white-light observing.

Riding the Solar Cycle

Solar activity varies with an 11-year cycle. As the cycle progresses, activity rises and falls, and with it the amount of detail visible on the Sun. At solar minimum, the Sun often appears nearly featureless, completely free of sunspots. At maximum, however, there can be hundreds of sunspots arranged in a half dozen or more groups and plenty of faculae. Obviously, the most exciting time to observe the Sun is in the years surrounding solar maximum. The last solar maximum was in 2000, and NOAA's Space Weather Prediction Center forecasts the next maximum for May 2013. So there's no better time than now to become a daylight astronomer!

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Plan for clear skies with our 2015 observing calendar! Each month features stunning astrophotography and sky scenes that illustrate the positions of the Moon and bright planets. Plus, keep track of important celestial events, such as eclipses, meteor showers, and conjunctions.